Albumin is a small globular protein found in several animal body fluids, including serum, cerebrospinal fluid (CSF), interstitial fluid, urine and amniotic fluid. Albumin is the major protein component of plasma, typically comprising more than half of total plasma protein. As such, albumin provides a suitable control in assays for determining analytes in test samples, such as human and animal plasma samples. For example, a series of albumin preparations can be used as reference controls for generation of a standard curve. Each albumin preparation of the series includes a defined concentration of the analyte. Typically, the series includes a preparation having a relatively low concentration of the analyte, a preparation having a relatively high concentration of the analyte, and several preparations having analyte concentrations between the low and high concentrations. To generate the standard curve, the entire series is tested alongside the test samples to generate a series of measurement data that corresponds to the various analyte concentrations evaluated. The standard curve, in turn, allows for determination of the concentration of the analyte in the test samples by extrapolation of a concentration from the standard curve based on measurement data for the test sample.
Because the standard curve provides the basis for extrapolating data for test samples, it is critical to include a series of reference control preparations that include an appropriate range of concentrations of the analyte of interest. The range of analyte concentrations used depends on the desired sensitivity for the assay. For example, the clinical chemistry of some analytes is such that it is desirable for an assay to have the ability to detect extremely low concentrations of the analyte in a test sample. In order for an assay to reliably detect these concentrations, the series of reference controls should include similar low concentrations of the analyte of interest, and desirably includes even lower concentrations.
Vitamin B12 and folate are two analytes for which extremely low serum concentrations can be clinically significant. Serum concentrations of folate may be clinically significant at levels of one to two nanograms per milliliter (ng/ml), while serum concentrations of Vitamin B12 may be clinically significant at levels of 100 picograms per milliliter (pg/ml) or less.
Accordingly, it is desirable to provide suitable reference controls for use in various analyte detection assays, including assays for determining Vitamin B12 and folate in test samples suspected of containing these analytes. Until now, albumin preparations did not have the desired extremely low concentrations of these analytes due at least in part to the presence of endogenous Vitamin B12 and folate in commercially available albumin.